Heat pumps and/or air conditioners essentially move thermal energy from one location to another, i.e., from a heat source to a heat sink. A heat pump varies from an air conditioner in that it can reverse the direction of thermal transfer, changing the source to the sink, and the sink to the source, thus being able to provide cooling in the summer and heating in the winter. The vapor compression refrigeration cycle, which is currently used for most cooling and heat pump systems and equipment, uses a circulating refrigerant as the medium which moves the heat through evaporation (heat absorption) and condensation (heat rejection), i.e., it absorbs and removes heat from a space to be cooled and subsequently rejects that heat elsewhere. The evaporation and condensation of the refrigerant typically takes place in two different heat exchangers called the evaporator and condenser, respectively. In a heat pump, the evaporator is switched to be a condenser and vice versa depending on whether cooling or heating is required. The efficiency of such a system, defined as the power input for the prime mover versus thermal energy transfer, is highly dependent on the temperature difference between the heat source and heat sink. This temperature difference between the heat source and the heat sink is referred to as “lift.” The greater the lift the lesser the efficiency of the system.
Geothermal, or ground source, heat pumps use the earth as a heat source/sink to improve the efficiency of the vapor compression refrigeration cycle by reducing the lift over conventional systems. The ground is a constant temperature of approximately 55° F. to 65° F. year round (depending on location). Typical heat pumps (air-to-air) use the outside air as the heat source/sink. Thus during the summer such air-to-air heat pumps attempt to reject heat to an approximate 91° F. sink and during the winter are attempting to absorb heat from a 0° F. heat source (temperatures dependant on location). Accordingly, the gained efficiency of having a 55° F. to 65° F. ground heat source/sink is apparent.
There are two basic types of known geothermal heat pumps, with some variations. A ground water heat pump, which is referred to as an open loop system, pumps water up from the ground and routes it to the heat pump condenser/evaporator and then either returns it to the ground in an injection well or runs it to surface water (storm). Alternatively, a ground-coupled heat pump, referred to as a closed loop system, uses a closed piping loop buried in the ground that moves the heat to and from the ground through a heat exchange process.
For both open and closed loop systems, a fluid, generally water for an open loop system and typically a water glycol mixture for the closed loop system, is piped directly to the one of the two heat exchangers in the heat pump. The heat exchanger's function alternates with the season, between being a condenser in the summer and being an evaporator in the winter.
A notable variation on open and closed loop systems is a standing column well. A standing column well is basically an open loop system that returns the water from the heat pumps to the same well that it is pumped out of. If the well(s) cannot keep up with system demand and maintain water well water supply temperature, then some of the water is diverted to surface (storm) water and not returned to the well(s). This is often referred to as “bleed,” and causes the well to bring the same amount of water out of the ground that is bled to storm. This water is at ground water temperature and will increase the wells capacity in times of high demand.
There are problems, however, associated with these known systems. In particular, although the open loop can be very effective and efficient, an adequate ground water yield (the amount of water that can be taken out of the ground on a sustained basis) is required, re-injection of the water into the earth is very difficult, stringent environmental permitting is often required, adding years and thousands of dollars to a project, and the effect on the local ground water is a concern. Indeed, consider that water is brought from deep below the earth's surface, run through several hundred feet of piping and mechanical equipment and then re-injected back deep into the earth, and it is understandable that various environmental agencies insist on oversight. Moreover, open loop systems often require that a test well be drilled to evaluate the yield capacity of the well before proceeding with the construction and implementation of the system.
While closed loop systems do not rely on ground water, do not have the same environmental concerns as open loops systems, do not require re-injection, and are more widely used with more reliable and expectable performance, they are also notably less efficient. Indeed, closed loop heat exchange with the earth is not efficient because it relies only on conduction with a limited radius in which to transfer heat into or from the ground.
As a hybrid of open and closed loop systems, known standing column wells have not achieved their goal of providing the benefits of both open and closed loop systems without the associated drawbacks. As will be readily appreciated, standing column wells are essentially open loop systems and, as such, still invoke all of the environmental considerations and issues that are applicable to open loop systems. In particular, these wells are generally very deep and, as such, can be difficult to drill. Although substantial yield is not supposed to be required, some yield for water bleed-off will be necessary if the system cannot keep up. As noted above, the water being bleed off to storm is not re-injected into the ground and, accordingly, can take about two years to make-up. As such, this type of standing column well may not be code compliant in some jurisdictions. Moreover, if adequate yield cannot be obtained because of water flow restrictions in the well, the water temperature may fall below freezing, potentially resulting in disastrous freeze-ups. As will be readily appreciated, a thermal fluid cannot be used because to protect the system from such freeze-ups because of the open loop nature of the system.
With the foregoing problems and concerns in mind, it is a general object of the present invention to provide a heat pump that combines the efficiency benefits of an open loop systems with the ease, stability and predictability of closed loop systems.